Street youth throughout the world struggle to survive without guidance from adult caretakers or refuge in safe home environments. They are at high risk for deficient development and a range of health problems including AIDS. These children and adolescents support themselves by performing odd jobs, petty theft, and prostitution. Exposure to male homosexuals and bisexuals by receptive anal intercourse is common in young children and heterosexual prostitution is almost universal in older girls living on the street. Sexually transmitted diseases are common and the incidence of HIV-I in sections and AIDS are increasing rapidly in these high-risk children. Valid information on the cognitive and behavioral antecedents and reinforcers of HIV risks among children and adolescents living "on the streets" is needed so that prevention programs can be planned. There is also an urgent need to undertake prevention studies contrasting different methods and intensities of intervention with street youth in relevant environmental contexts. To address these issues, a multidisciplinary research team of pediatricians, psychologists, sociologists, anthropologists, and epidemiologists experienced in AIDS research has been formed. In this grant proposal, we outline integrated multi-method descriptive analytic, and preventive intervention studies to be conducted at two sites (the urban slums of Belo Horizonte, Brazil and urban Baltimore, Maryland) where unfortunate "natural experiments" involving both normative developmental processes and sub-cultural practices are leading to a high incidence of HIV-I infections among children and adolescents. The Brazilian site is unique in that governmental, University, health, and youth care organizations have invited the Johns Hopkins University research team to collaborate in longitudinal assessment of risk factors for HIV-I seropositivity, descriptive behavioral studies and controlled prevention research in a concerted effort to help several million street youth.